There is an increasing demand for a high speed, high resolution and low price non-impact printer. Among the several kinds of non-impact printers on the market, laser printers are most practical because their advantage of high printing speed. However, there is a desire to decrease the size and manufacturing cost of such laser printers.
Recently, an optical printer utilizing a ferroelectric liquid crystal light shutter device has been proposed for experimental use. The ferroelectric liquid crystal light shutter device comprises a pair of glass substrates, a predetermined number N of transparent scanning electrodes provided on the inner surface of one of the glass substrates to be divided into, for instance, groups of four, N/4 signal electrodes provided on the inner surface of the other glass substrate each facing the corresponding scanning electrodes, alignment layers respectively covering the scanning electrodes and signal electrodes, spacers positioned between the glass substrates to provide a predetermined facing distance therebetween, polarizers provided on the respective outer surfaces of the glass substrates, and a layer of ferroelectric liquid crystal, exhibiting a chiral smectic phase, which is described in detail later, contained between the alignment layers. The layer of ferroelectric liquid crystal has a thickness of, for instance, 2.5.mu.m, that is, less than a helical pitch of its molecules. In other words, the predetermined facing distance of the spacers is selected to be that value. In general, the helical pitch is in the range of 3.mu.m to 10.mu.m.
In operation, the scanning electrodes divided into groups of four are addressed with 1/4 duty ratio multiplex driving so that scanning signals are applied thereto in a predetermined scanning time. The signal electrodes are driven in accordance with image data signals. As a result, light shutter elements each including an addressed scanning electrode and a driven signal electrode are turned on to pass light radiating from a light source whereby a photoconductor drum is exposed to provide a latent image thereon.
In the ferroelectric liquid crystal light shutter device, high speed response as fast as, for instance, 0.18ms is obtained as a result of the ferroelectric liquid crystal employed therein. This high speed switching response of ferroelectric liquid crystals is described in "Submicrosecond Bistable Electro-optic Switching in Liquid Crystals", authored by Noel A. Clark et al., at pages 899 to 901 of "Appl. Phys. Lett. 36(11), 1 June 1980".
In the proposed ferroelectric liquid crystal light shutter device, however, bistable switching is not as ideal as expected because the electric field reverse to the polarity of electric field for selecting light shutter elements is applied to ferroelectric liquid crystal portions each corresponding to the non-addressed scanning electrode and the selected signal electrode, so that it is difficult to maintain formerly memorized states in all of the corresponding light shutter elements. Such a reverse electric field is applied to all of light shutter elements including selected signal electrodes in the former half scanning time to erase former states of the light shutter elements to be selected in the latter half scanning time. Such a tendency of not maintaining the selected states of the non-addressed light shutter elements becomes particularly acute when the duty ratio for multiplex driving is higher because there is more time during which the scanning electrodes are not addressed, so that the influence of the reverse electric field becomes greater therein. Consequently, image contrast is decreased, so that the advantages of utilizing a ferroelectric liquid crystal are partially last.
Further, each scanning time consists of the former half time during which memorized states of the light shutter elements are erased and the latter half time during which some of the light shutter elements are selected, so that the scanning time necessary for a scanning electrode is twice the time necessary for a response of ferroelectric liquid crystal in light shutter elements so that the advantage of high speed ferroelectric liquid crystal response is not fully realized.